js/wasm: introduce scope for JS object

[romaindoumenc]

This changes allow to attach a JS object (e.g. browser component) to a
js/wasm Go program. This creates a scope to only capture relevant events,
enabling Web applications mixing Go / native components.

An example use is discussed in the GitHub issue.

Implementation-wise, this contains two changes:

• on the JS side the go object gets a new field 'scope' that is checked
• on the Go side, in package syscall/js, func Scope() Value returns
  go.scope when it has been provided, or else window/global.

A small test uses a dummy object to perform a simple get/set action on this
new field.

Fixes #56084

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